As to electrophotographic photoreceptors using photoconductive materials as photosensitive materials, the photoreceptors have previously been known in which inorganic photoconductive materials such as selenium and zinc oxide and various organic photoconductive materials are used as the photoconductive materials. Of these photoreceptors, a function separation type electrophotographic photoreceptor in which a photosensitive layer is separated into a charge generating layer and a charge transporting layer has rapidly advanced in recent years, because the sensitivity which is a major factor for photoreceptors using conventional organic photoconductive materials can be significantly improved.
As means for improving the sensitivity of a photoreceptor, it is reported that the sensitivity increases with a decrease in the ionization potential IPt1 of the charge transporting layer when the ionization potential IPg1 of the charge generating layer is higher than the ionization potential IPt1 of the charge transporting layer in the case of the negative charge type photoreceptors (refer to Chemistry and Industry (Kagaku To Kogyo), No.34, Vol. 7, 489-492 (1981)). Further, JP-A-60-207142 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses that a photoreceptor is highly sensitized by the use of a phthalocyanine powder and a charge transporting material having an ionization potential of 6.1 to 6.7. Furthermore, JP-A-1-219752 discloses that a photoreceptor is highly sensitized by the use of a charge generating layer and a charge transporting layer in which the difference between the ionization potential IPt1 of the charge transporting layer and the ionization potential IPg1 of the charge generating layer (IPt1-IPg1) is .+-.0.14 to 0.26 eV, namely the ionization potential of the charge transporting layer is higher than that of the charge generating layer by a value within the specified range.
In the method proposed in the above-described Chemistry and Industry, however, the ionization potential of the charge generating material is established more highly than that of the charge transporting material. Hence, although the residual potential of the photoreceptor decreases to improve the sensitivity, the lowering rate of the surface potential of the photoreceptor until light irradiation after initial charging, namely the dark decay ratio, increases at the same time. As a result, the photoreceptor has the disadvantage that the contrast potential after light irradiation is insufficient.
The photoreceptors produced by the methods described in JP-A-60-207142 and JP-A-1-219752 exhibit a sensitivity good to some extent, but the residual potential increases, so that the photoreceptors have the disadvantage that the contrast potential after light irradiation is insufficient, also.